1. Field of the Invention
The present invention relates to a compact imaging apparatus, and in particular to a zoom lens most suitably used with a digital still camera.
2. Description of the Related Art
A conventional zoom lens such as a zoom lens of inner focus type for the video camera is configured of a lens system including, arranged from the object side, four lens groups having positive, negative, positive and positive refracting power in that order in many cases.
The zoom lens a shown in FIG. 1, for example, is configured of four lens groups for a total of nine lenses, including, from the object side, a first lens group Gr1 having three lenses with a positive refractive index as a whole, a second lens group Gr2 having three lenses with a negative refractive power as a whole adapted to move along the optical axis for magnification change, a third lens group Gr3 having a single lens with a positive refracting power and a fourth lens group Gr4 having two lenses with a positive refracting power as a whole.
The conventional zoom lens for the video camera, however, has the following problem if used for the still camera.
The following imaging quality is required of the video camera used for picking up a moving image or image sequences and the still camera used for taking a static image. In a still camera for taking a static image and often printing it out on the paper, the image printed out is observed in detail up to the high-frequency band, so that many pixels are required of the imaging apparatus and a high modulation transfer function (MTF) value is required up to a high-frequency band for the lens system performance. Assuming that a video camera using a 1/4-inch CCD having a pixel pitch of 5.5 .mu.m and MTF of 50 line.multidot.pair/mm, for example, the pixel pitch is 3.125 .mu.m and the MTF of 90 line.multidot.pair/mm for a digital still camera using the same 1/4-inch CCD. In the case where the number of pixels of the digital still camera exceeds one million in this way, it is difficult to maintain the same MTF value at a high spatial frequency.
Various zoom lenses having a lens configuration different from that of the zoom lens a shown in FIG. 1 for the video camera to pick up image sequences have been proposed. The main object of these lenses, however, is to reduce the size and there have been very few cases in which the basic configuration of the lens system is reviewed for the purpose of improving the image quality.
The problem of the conventional zoom lens a in respect of the image quality has been that of correction of the chromatic aberration. Specifically, the chromatic aberration on the axis at the wide angle edge of the zoom lens is effectively corrected in the third or fourth lens group where the light fluxes widen. In the zoom lens a described above, therefore, the third lens group Gr3 is configured of a single lens for reducing the cost and the chromatic aberration on the axis is corrected by the fourth lens group Gr4 in such a manner as to assure the balance of the whole lens system. The fourth lens group Gr4, in which the chromatic aberration is corrected when the height of the main light ray increases, is effective also for the correction of the chromatic aberration of magnification.
The fourth lens group Gr4 moves along a convex locus toward the object when the second lens group Gr2 is moved linearly for magnification change. In the case where an arrangement is made to assure the highest effect of correcting the chromatic aberration on the axis at a position farthest from the image surface and to assure a satisfactory effect at the wide angle edge, the effect of correcting the chromatic aberration on the axis becomes excessive at the position described above, thereby leading to the blur of the color having a short wavelength. As for the chromatic aberration of magnification, on the other hand, once the fourth lens group Gr4 reaches a position farthest from the image surface, the height of the main light ray is lowered so that the color of short wavelength tends to blur inward of the image. Mainly for the reason of these variations of the effect of chromatic aberration correction described above, the demand for the quality of a static image has failed to be met.
The zoom lens a is configured to maintain the balance of various aberrations on the wide angle side by offsetting the various aberrations of the first lens group Gr1 and the second lens group Gr2 at the telephotographic edge. As for the chromatic aberration, however, the blur of the color of short wavelength generated out of the axis from the joint surface of lenses constituting the first lens group Gr1 cannot be offset by the second lens group Gr2. This constitutes a factor for determining the image quality at the telephotographic edge. Also, in the case where the first lens group Gr1 is configured of three lenses like the zoom lens a, the freedom of the radius of curvature of the joint surface is limited by the balance between the freedom of selecting an optical glass to be used and the correction of other aberrations, thereby making it impossible to effectively correct the blur of the color of short wavelength at the telephotographic edge.